KABUL -- Afghanistan's Interior Ministry says it has discovered that the policewoman who shot dead a U.S. civilian police trainer at Kabul's police headquarters on December 24 is not an Afghan national, as they originally thought, but is instead an Iranian citizen.

Interior Ministry spokesman Sidiq Sidiqi told journalists in Kabul on December 25 that the woman detained for the killing -- Afghan Interior Ministry police Sergeant Nargis -- obtained Afghan citizenship about a decade ago on the basis of false documents that were provided by her husband.

According to Sidiqi, the discovery means Nargis, a 33-year-old mother of four who has worked for the Interior Ministry's gender rights section for the past two years, is still legally a citizen of Iran where she was born.

"By marrying an Afghan, she got an Afghan identity card, as her husband wanted her to, and based on the assurances of her husband's identification, documents proving he is an Afghan citizen, she received her [Afghan] identity card, which was illegal," Sidiqi said. "Our investigation shows the way she obtained her identity card was illegal. She got it through illegal means. This is how she entered the Afghan police force.”

Sidiqi said Nargis has displayed "unstable behavior" since her arrest, but a joint Afghan-NATO investigation has not revealed any links with militants so far.

Afghan authorities also said on December 25 that Nargis's husband has also been detained in the case and was being questioned as part of the investigation.

First 'Insider' Attack By A Woman

Afghan and NATO authorities had described the attack on December 24 against the U.S. civilian contractor as the first "insider" attack by a member of Afghanistan's security forces to be committed by a woman.

The man she killed -- 49-year-old Joseph Griffin from the U.S. state of Georgia -- was employed by the U.S. private security firm DynCorp International.

He was working in Kabul to train members of the Afghan National Police, who are, along with the Afghan National Army, taking over security operations from NATO-led foreign forces who are due to withdraw from Afghanistan by the end of 2014.

DynCorp International said on its website that Griffin was a veteran of the U.S. military who had earlier worked with law-enforcement agencies in the United States.

Brigadier General John Madower, a spokesman for the NATO-led International Security Assistance Force (ISAF), said Nargis was dressed as a member of Afghanistan's uniformed police force when she carried out the attack inside the compound of Kabul's heavily guarded police headquarters.

Kabul Deputy police chief General Dawood Amin confirmed that she was a "longtime employee" of the Interior Ministry.